This invention relates to the spectrophotometric analysis of blood parameters.
Blood parameters or indices such as hematocrit and transvascular fluid exchange, provide important clinical information. For example, there are a number of clinical conditions, e.g. renal failure and massive fluid overload, which require fluid removal by natural means or the use of artificial devices, and in which it would be beneficial to be able to achieve an optimal rate of fluid removal.
Commonly used clinical approaches for fluid removal include an extracorporeal circuit. Because the body tissues and the blood stream are in direct communication, fluid removal from the blood results in an imbalance of forces which favors fluid movement from the tissues toward the blood stream. Continuous fluid removal from the blood stream therefore results in continuous mobilization from the tissues, which is the ultimate purpose of the therapy.
However, if fluid removal from the blood stream is induced at a rate faster than fluid can be mobilized from the tissues, patients can become hypotensive (shock). This complication is difficult to predict even if blood pressure is followed continuously. Presumably, hemodynamic compensatory mechanisms succeed in maintaining normal blood pressure even in the presence of hypovolemia until decompensation ensues and circulatory shock develops. To prevent this possibility, fluid removal may be induced at a very slow rate; however, this approach is disadvantageous. It is therefore desirable to achieve an optimal rate of fluid removal: the fastest rate possible without inducing hypotension. In this way, dialysis may be optimized.
Fluid removal from the blood or fluid mobilization from the tissues into the blood results in change in the hematocrit, the fraction of red cell volume relative to total blood. Change in hematocrit induces change in the optical properties of blood.
Because red cells contain hemoglobin, a naturally occurring pigment with a continuous spectrum of light absorption throughout the ultraviolet, visible, and infrared ranges, changes in hematocrit result in changes in blood hemoglobin concentration and therefore in changes in light transmission across a blood column. It has been established that hematocrit may be correlated with light transmission changes across a flowing blood column, and measurements of hematocrit may be used to derive measurements of other blood indices.
As illustrated by U.S. Pat. Nos. 3,830,569 to Meric, 4,243,883 to Schwartzmann and 4,303,336 to Cullis, a spectrophotometric device for measuring blood indices including a suitable light source and photodetector is known. The light source may be a laser with a collimating lens mounted in front of the laser, and the light source may be remote from or proximate to the blood. As shown by Meric, multiple detectors in combination with a centrally disposed, light trap have been used, and a shield provided with windows may be disposed in the light path.
Also known as exemplified by U.S. Pat. No. 4,484,135 to Ishibara et al, is hematocrit determination by blood resistivity measurement. This patent criticizes measurements deriving hematocrit from blood cell count and mean blood cell volume, for using diluted blood if the concentration of the electrolyte and protein in the diluted blood has been changed.
As illustrated by J. Appl. Physiol., 62(1): 364 (1987), light transmissive, plastic tubing through which blood is circulated, may be placed between light sources and light detectors disposed generally opposite from the light sources. High molecular weight, Blue Dextran may be used as an impermeable, reference tracer. As exemplified by J. Appl. Physiol., 69(2): 456 (1990), to ensure detection of a widely scattered light beam, detectors connected in parallel to a photodetector circuit may be used. A separate detector may measure light fluctuations. Diluted perfusate is described.
Unless taken into account, measurement artifacts may induce error in determining blood parameters from optical properties of the blood. One well-known artifact results from changes in the degree of oxygen saturation of hemoglobin (SaO.sub.2). It has been found that this artifact may be avoided by selecting a specific wavelength or isobestic point at which the hemoglobin oxygen content does not influence light transmission. Isobestic points are known in the IR range (approximately 814 nm), in the green range (approximately 585 and 555 nm), and in the UV range.
As illustrated by U.S. Pat. Nos. 4,745,279 to Karkar et al, 4,776,340 to Moran et al, 5,048,524 to Bailey, and 5,066,859 to Karkar et al, additional detectors have been used to compensate for various measurement artifacts, in particular for variation in intensity of light entering the blood stream. In Karkar '279, a detector for diffused light is used in combination with a compensating detector directly illuminated by another light source. In Moran, a far detector and a near detector are used, and the ratio of the separately amplified signals is determined. Similar to Moran is U.S. Pat. No. 5,149,503 to Kohno et al, in which an additional emitter location and signal ratios are used.
In Karkar '859, a pair of far field detectors for determining hematocrit and a pair of near field detectors for obtaining oxygen saturation measurements are used. The far field detectors are spaced equidistant from a light emitting fiber, and one of the far field detectors is used to compensate for the signal detected by its paired fiber.
However, the correlation between blood parameters and optical properties of blood needs to be improved. Accordingly, there is a need for improvement in using the optical properties of blood to determine blood indices. Beneficially, applications relating to monitoring of bodily fluid shifts would be facilitated. For instance, monitoring of fluid removal, including as appropriate the use of feedback, so as to achieve an optimal rate of removal, would be facilitated. In connection therewith, drug therapy for, for instance, inducing fluid reabsorption or restoring vascular membrane permeability may be used.